1. Field of the Invention
The present invention relates to a CDMA system wireless communication apparatus and wireless communication method, in which demodulation is carried out by a pilot coherent detection.
2. Related Art
Recently, in the Cellular system such as automobile telephones and portable telephones, etc., an effective utilization technique of frequencies which is capable of securing a capacity of a number of subscribers on limited frequency bands has become important.
A code division multiple access (CDMA) system has been noted as one of multiple access systems which is able to effectively utilize frequencies. The CDMA system is also called "Spread Spectrum Multiple Access (SSMA) system", which is able to achieve excellent communication quality by a wide band feature and a correlational property by codes of pseudorandom noise (PN), etc.
A land mobile communication system in which the CDMA system is employed, is disclosed in, for example, U.S. Pat. No. 4,901,307. As one of the CDMA systems, there is a direct spread system in which pseudorandom noise codes (hereinafter called "spread code") are multiplied onto transmission signals.
In a case where the direct spread system is employed, it is possible to increase a diversity effect by receiving in rake and synthesizing the multi-path components in the maximum ratio. Rake receiving is disclosed in, for example, U.S. Pat. No. 5,109,390.
Hereinafter, a description is given of the outline of CDMA wireless communication system in which the direct spread system is employed, with reference to FIG. 1. FIG. 1 is a block diagram showing the configuration of CDMA wireless communication system using the direct spread system.
As shown in FIG. 1, the transmission side of CDMA wireless communication system is mainly composed of a data modulating section 1 for modulating the transmission data, a spread code generating section 2 for generating spread codes, a spread modulating section 3 for multiplying spread codes onto transmission signals, and a transmission antenna 4 for wirelessly transmitting transmission signals.
Furthermore, the receiving side of the same system is mainly composed of a receiving antenna 5 for receiving signals wirelessly transmitted from the transmission side, a synchronous acquisition section 6 for generating spread codes at the same timing as that of those generated by the spread code generating section 2, a despreading section 7 for multiplying spread codes onto receiving signals, and a data demodulating section 8 for demodulating despread receiving signals.
In a CDMA wireless communication system constructed as described above, transmission data such as voice data is modulated by the data modulating section 1, and the modulated signals are narrow-band signals having only a band width necessary for transmission as shown at (a).
Furthermore, the band width of spread codes generated by the spread code generating section 2 is sufficiently wide in comparison with the band width of signals modulated by the data modulating section 1 as shown at (b).
The signals modulated by the data modulating section 1 are transmitted by the transmission antenna 4 with the spread codes thereof multiplied by the spread modulating section 3.
Signals received by the receiving antenna 5 are multiplied again by the despreading section 7 with spread codes generated by the synchronous acquisition section 6 and are despread from a wide band to a narrow band.
Interference resulting from other users and thermal noise, etc. are caused to add on the transmitted signals in the process to the receiving side as shown at (c). However, since spread codes have sufficiently small correlation with the interference, the interference are reduced by the despreading section 7 as shown at (d) to cause the received data to be detected.
Here, in the CDMA wireless communication system such as a down link of Cellular system, etc., there is such a type where pilot signals which are already known to each user are always or periodically transmitted through a pilot channel using the pilot channel along with a data channel for transmitting data with respect to the users.
At the receiving side of the CDMA wireless communication system, using signals (hereinafter called "pilot signals") transmitted through the pilot channel, several processes such as synchronous acquisition, tracking, estimation of multi-path condition, coherent detection, frequency synchronization, hand-off, etc. are carried out. Of them, coherent detection process of signals (hereinafter called "data signals") transmitted from the data channel using the pilot signals is called "pilot coherent detection".
Hereinafter, a description is given of the pilot coherent detection in a conventional wireless communication apparatus, with reference to a block diagram shown in FIG. 2. Furthermore, FIG. 2 corresponds to the synchronous acquisition section 6 and the despreading section 7 of FIG. 1.
As shown in FIG. 2, the conventional wireless communication apparatus despreads receiving signals using pilot spread codes, and the apparatus is mainly composed of a pilot despreading section 11 for detecting pilot signals, an averaging section 12 for averaging the detected pilot signals as described later, a data despreading section 13 for despreading the received signals using spread codes for data and for detecting the data signals, and a coherent detecting section 14 for correcting the phase shift of data signals using the averaged pilot signals.
With regard to signals received by the wireless communication apparatus constructed as described above, pilot signals are detected by being despread with spread codes for pilot at the pilot despreading section 11 and data signals are detected by being despread with spread codes for data at the data despreading section 13.
The detected pilot signals are averaged by the averaging section 12, wherein the phase shift of the data signals is corrected by the coherent detecting section 14 on the basis of the averaged pilot signals.
Next, a description is given of the averaging of pilot signals, wherein it is assumed that the spread is carried out by quadrature phase shift keying system (QPSK system).
Pilot signals outputted from the pilot despreading section 11 are expressed by formula 1 shown below; EQU I.sub.0 (t)=A.sub.0.multidot.cos .phi.(t)+n.sub.0.sup.I (t) EQU Q.sub.0 (t)=A.sub.0.multidot.sin .phi.(t)+n.sub.0.sup.Q (t) Formula 1
where A.sub.0 shows the level of pilot signals, .phi.(t) shows a carrier phase offset, and n.sub.0.sup.I (t), n.sub.0.sup.Q (t) show interference. The interference includes interference of other multi-path waves, interference of channels other than the pilot channel, and interference of other cells., etc.
Pilot signals are averaged by the averaging section 12 in the period of time for which the values of cos .phi.(t), sin .phi.(t), etc. are regarded to be constant. By this averaging, the interference are decreased, the level A.sub.0, and values of cos .phi.(t) and sin .phi.(t) of the pilot signals are obtained. Furthermore, the data signals outputted from the data despreading section 13 for user k are expressed by formula 2 described below; EQU I.sub.k (t)=A.sub.k.multidot.d.sub.k (t).multidot.cos .phi.(t)+n.sub.k.sup.I (t) EQU Q.sub.k (t)=A.sub.k.multidot.d.sub.k (t).multidot.sin .phi.(t)+n.sub.k.sup.Q (t) Formula 2
A.sub.k indicates the level of spread code of user k, d.sub.k (t) indicates the data of user k, and n.sub.k.sup.I (t), n.sub.k.sup.Q (t) indicates interference.
If it is assumed that there is no interference in the pilot signals and data signals after averaging, that is, if it is assumed that interference n.sub.0.sup.I (t), n.sub.0.sup.Q (t), n.sub.k.sup.I (t), n.sub.k.sup.Q (t) in formulas 1 and 2 are zero (0), the carrier phase offset is corrected by an inner product calculation at the coherent detecting section 14, wherein demodulation signals expressed by the following formula 3 can be obtained. EQU I.sub.0 (t).multidot.I.sub.k (t)+Q.sub.0 (t).multidot.Q.sub.k (t)=A.sub.0.multidot.A.sub.k.multidot.d.sub.k (t) Formula 3
Herein, in wireless lines such as mobile communications, the level A.sub.0 of pilot signals and phase offset .phi.(t) change in line with movement of a mobile station, a frequency shift occurs due to fading in addition to carrier frequency offset of a local oscillator at the receiving side.
However, the abovementioned conventional wireless communication apparatus is not able to correct the phase shift of date signals unless the cycle of averaging is in the period of time for which the value of cos .phi.(t) and sin .phi.(t) of formula 1 can be regard to be constant.
Since the cycle of averaging needs much shorter than the carrier frequency offset or the frequency shift due to fading, the abovementioned conventional wireless communication apparatus is not able to sufficiently decrease the interference added onto the received signals in a case where the carrier frequency offset or the frequency shift occurs due to fading. As result, the demodulation signals are deteriorated after pilot coherent detection is performed, and the receiving quality is lowered.